Guide to Aluminum Welding

Summary:

Aluminum is a lightweight, soft, low strength metal which can easily be cast, forged, machined, formed and welded. Unless alloyed with specific elements, it is suitable only in low temperature applications. Aluminum is readily joined by welding, brazing, and soldering. In many instances, aluminum is joined with the conventional equipment and techniques used with other metals. However, specialized equipment or techniques may sometimes be required. The alloy, joint configuration, strength required, appearance, and cost are factors dictating the choice of process. Each process has certain advantages and limitations.

Color

Aluminum is light gray to silver in color, very bright when polished, and dull when oxidized.

Characteristics

A fracture in aluminum sections shows a smooth, bright structure. Aluminum gives off no sparks in a spark test, and does not show red prior to melting. A heavy film of white oxide forms instantly on the molten surface. Aluminum is light in weight and retains good ductility at subzero temperatures. It also has high resistance to corrosion, good electrical and thermal conductivity, and high reflectivity to both heat and light. Pure aluminum melts at 1220ºF (660ºC), whereas aluminum alloys have an approximate melting range from 900 to 1220ºF (482 to 660ºC). There is no color change in aluminum when heated to the welding or brazing range.

Its combination of light weight and high strength make aluminum the second most popular metal that is welded.

Single Wire MIG Aluminum Welding

Aluminum Vs. Steel Welding

One reason aluminum is different from steels when welding is that
it does not exhibit color as it approaches its melting temperature until
it is raised above the melting point, at which time it will glow a dull
red. When soldering or brazing aluminum with a torch, flux is used. The
flux will melt as the temperature of the base metal approaches the
temperature required. The flux dries out first, and melts as the base
metal reaches the correct working temperature. When torch welding with
oxyacetylene or oxyhydrogen, the surface of the base metal will melt
first and assume a characteristic wet and shiny appearance. (This aids
in knowing when welding temperatures are reached.) When welding with gas
tungsten arc or gas metal arc, color is not as important, because the
weld is completed before the adjoining area melts.

Molten Aluminum Filler

Welding Properties and Alloys

Aluminum and aluminum alloys can be satisfactorily welded by metal-arc, carbon-arc, and other arc welding processes. Pure aluminum can be alloyed with many other metals to produce a wide range of physical and mechanical properties. The means by which the alloying elements strengthen aluminum is used as a basis to classify alloys into two categories: nonheat treatable and heat treatable. Wrought alloys in the form of sheet and plate, tubing, extruded and rolled shapes, and forgings have similar joining characteristics regardless of the form. Aluminum alloys are also produced as castings in the form of sand, permanent mold, or die castings. Substantially the same welding, brazing, or soldering practices are used on both cast and wrought metal. Die castings have not been widely used where welded construction is required. However, they have been adhesively bonded and to a limited extent soldered. Recent developments in vacuum die casting have improved the quality of the castings to the point where they may be satisfactorily welded for some applications.

The principal advantage of using arc welding processes is that a highly concentrated heating zone is obtained with the arc.

For this reason, excessive expansion and distortion of the metal are prevented.

Aluminum possesses a number of properties that make welding it
different than the welding of steels. These are: aluminum oxide surface
coating; high thermal conductivity; high thermal expansion coefficient;
low melting temperature; and the absence of color change as temperature
approaches the melting point. The normal metallurgical factors that
apply to other metals apply to aluminum as well.

Aluminum is an active metal which reacts with
oxygen in the air to produce a hard, thin film of aluminum oxide on the
surface. The melting point of aluminum oxide is approximately 3600ºF
(1982ºC) which is almost three times the melting point of pure aluminum
(1220ºF (660ºC)). In addition, this aluminum oxide film absorbs moisture
from the air, particularly as it becomes thicker. Moisture is a source
of hydrogen, which causes porosity in aluminum welds. Hydrogen may also
come from oil, paint, and dirt in the weld area. It also comes from the
oxide and foreign materials on the electrode or filler wire, as well as
from the base metal. Hydrogen will enter the weld pool and is soluble in
molten aluminum. As the aluminum solidifies, it will retain much less
hydrogen. The hydrogen is rejected during solidification. With a rapid
cooling rate, free hydrogen is retained within the weld and will cause
porosity. Porosity will decrease weld strength and ductility, depending
on the amount.

Welding Rods

Stick welding aluminum (aluminum welding rods) are available at a thickness that is approximately 1/8" of steel. It is an excellent choice for repairing tanks and pipes in the field. Also a good choice when working in windy conditions. It is not for precise work.

The downside of using aluminum welding rods is the need for a substantial amount of practice. There is also a flux issue. the flux burns aggressivley and is hard to remove. It also burns through paint.

There are superior alternatives to aluminum welding rods such as wire feed welding.

Aluminum Alloy Numbering

Many alloys of aluminum have been developed. It is important to know
which alloy is to be welded. A system of four-digit numbers has been
developed by the Aluminum Association, Inc., to designate the various
wrought aluminum alloy types.

This system of alloy groups is as follows:

1XXX series. These are aluminums of 99 percent or higher purity
which are used primarily in the electrical and chemical industries.

2XXX series. Copper is the principal alloy in this
group, which provides extremely high strength when properly heat
treated. These alloys do not produce as good corrosion resistance and
are often clad with pure aluminum or special-alloy aluminum. These
alloys are used in the aircraft industry.

3XXX series. Manganese is the major alloying
element in this group, which is non-heat-treatable. Manganese content is
limited to about 1.5 percent. These alloys have moderate strength and
are easily worked.

4XXX series. Silicon is the major alloying element
in this group. It can be added in sufficient quantities to substantially
reduce the melting point and is used for brazing alloys and welding
electrodes. Most of the alloys in this group are non-heat-treatable.

5XXX series. Magnesium is the major alloying
element of this group, which are alloys of medium strength. They possess
good welding characteristics and good resistance to corrosion, but the
amount of cold work should be limited.

6XXX series. Alloys in this group contain silicon
and magnesium, which make them heat treatable. These alloys possess
medium strength and good corrosion resistance.

7XXX series. Zinc is the major alloying element in
this group. Magnesium is also included in most of these alloys.
Together, they form a heat-treatable alloy of very high strength, which
is used for aircraft frames.

Cleaning

Since aluminum has a great affinity for oxygen, a film of oxide is always present on its surface. This film must be removed prior to any attempt to weld, braze, or solder the material. It also must be prevented from forming during the joining procedure. In preparation of aluminum for welding, brazing, or soldering, scrape this film off with a sharp tool, wire brush, sand paper, or similar means. The use of inert gases or a generous application of flux prevents the forming of oxides during the joining process.

Aluminum and aluminum alloys should not be cleaned with caustic soda or
cleaners with a pH above 10, as they may react chemically. The aluminum oxide film must be removed prior to welding. If it is
not completely removed, small particles of un-melted oxide will be
trapped in the weld pool and will cause a reduction in ductility, lack
of fusion, and possibly weld cracking.

The aluminum oxide can be removed by mechanical,
chemical, or electrical means. Mechanical removal involves scraping with
a sharp tool, sandpaper, wire brush (stainless steel), filing, or any
other mechanical method. Chemical removal can be done in two ways. One
is by use of cleaning solutions, either the etching types or the
nonetching types. The nonetching types should be used only when starting
with relatively clean parts, and are used in conjunction with other
solvent cleaners. For better cleaning, the etching type solutions are
recommended, but must be used with care. When dipping is employed, hot
and cold rinsing is highly recommended. The etching type solutions are
alkaline solutions. The time in the solution must be controlled so that
too much etching does not occur.

Chemical Cleaning

Chemical cleaning includes the use of welding fluxes.
Fluxes are used for gas welding, brazing, and soldering. The coating on
covered aluminum electrodes also maintains fluxes for cleaning the base
metal. Whenever etch cleaning or flux cleaning is used, the flux and
alkaline etching materials must be completely removed from the weld area
to avoid future corrosion.

Electrical Oxide Removal System

The electrical oxide removal system uses cathodic
bombardment. Cathodic bombardment occurs during the half cycle of
alternating current gas tungsten arc welding when the electrode is
positive (reverse polarity). This is an electrical phenomenon that
actually blasts away the oxide coating to produce a clean surface. This
is one of the reasons why AC gas tungsten arc welding is so popular for
welding aluminum.

Since aluminum is so active chemically, the oxide film
will immediately start to reform. The time of buildup is not extremely
fast, but welds should be made after aluminum is cleaned within at least
8 hours for quality welding. If a longer time period occurs, the
quality of the weld will decrease.

Thermal Conductivity

Aluminum has a high thermal conductivity and low melting
temperature. It conducts heat three to five times as fast as steel,
depending on the specific alloy. More heat must be put into the
aluminum, even though the melting temperature of aluminum is less than
half that of steel. Because of the high thermal conductivity, preheat is
often used for welding thicker sections. If the temperature is too high
or the time period is too long, weld joint strength in both
heat-treated and work-hardened alloys may be diminished. The preheat for
aluminum should not exceed 400ºF (204ºC), and the parts should not be
held at that temperature longer than necessary. Because of the high heat
conductivity, procedures should utilize higher speed welding processes
using high heat input. Both the gas tungsten arc and the gas metal arc
processes supply this requirement.

The high heat conductivity of
aluminum can be helpful, since the weld will solidify very quickly if
heat is conducted away from the weld extremely fast. Along with surface
tension, this helps hold the weld metal in position and makes
all-position welding with gas tungsten arc and gas metal arc welding
practical.

The thermal expansion of aluminum is twice that of
steel. In addition, aluminum welds decrease about 6 percent in volume
when solidifying from the molten state. This change in dimension may
cause distortion and cracking.

Aluminum Plate Welding

For aluminum plate welding, because of the difficulty of controlling the arc, butt
and fillet welds are difficult to produce in plates less than 1/8 in.
(3.2 mm) thick. When welding plate heavier than 1/8 in. (3.2 mm), a
joint prepared with a 20 degree bevel will have strength equal to a weld
made by the oxyacetylene process. This weld may be porous and
unsuitable for liquid-or gas-tight joints. Metal-arc welding is,
however, particularly suitable for heavy material and is used on plates
up to 2-1/2 in. (63.5 mm) thick.

Current and polarity settings

The current and
polarity settings will vary with each manufacturer's type of electrodes.
The polarity to be used should be determined by trial on the joints to
be made.

Plate edge preparation

In general, the design of
welded joints for aluminum is quite consistent with that for steel
joints. However, because of the higher fluidity of aluminum under the
welding arc, some important general principles should be kept in mind.
With the lighter gauges of aluminum sheet, less groove spacing is
advantageous when weld dilution is not a factor. The controlling factor
is joint preparation. A specially designed V groove is excellent
where welding can be done from one side only and where a smooth,
penetrating bead is desired. The effectiveness of this particular design
depends upon surface tension, and should be applied on all material
over 1/8 in. (3.2 mm) thick. The bottom of the special V groove must be
wide enough to contain the root pass completely. This requires adding a
relatively large amount of filler alloy to fill the groove.

Excellent
control of the penetration and sound root pass welds are obtained. This
edge preparation can be employed for welding in all positions. It
eliminates difficulties due to burn-through or over-penetration in the
overheat and horizontal welding positions. It is applicable to all
weldable base alloys and all filler alloys.

Aluminum MIG Welding

Fully-Automatic Single Wire MIG Welding

Gas Metal-Arc (MIG) Welding (GMAW)

This fast, adaptable process is used with direct current re-verse
polarity and an inert gas to weld heavier thicknesses of aluminum
alloys, in any position, from 1/016 in. (1.6 mm) to several inches
thick. TM 5-3431-211-15 describes the operation of a typical MIG welding
set.

Shielding Gas

Precautions should be taken to
ensure the gas shield is extremely efficient. Welding grade argon,
helium, or a mixture of these gases is used for aluminum welding. Argon
produces a smother and more stable arc than helium. At a specific
current and arc length, helium provides deeper penetration and a hotter
arc than argon. Arc voltage is higher with helium, and a given change in
arc length results in a greater change in arc voltage. The bead profile
and penetration pattern of aluminum welds made with argon and helium
differ. With argon, the bead profile is narrower and more convex than
helium. The penetration pattern shows a deep central section. Helium
results in a flatter, wider bead, and has a broader under-bead
penetration pattern. A mixture of approximately 75 percent helium and 25
percent argon provides the advantages of both shielding gases with none
of the undesirable characteristics of either. Penetration pattern and
bead contour show the characteristics of both gases. Arc stability is
comparable to argon. The angle of the gun or torch is more critical when
welding aluminum with inert shielding gas. A 30º leading travel angle
is recommended. The electrode wire tip should be oversize for aluminum.
Table 7-21 provides welding procedure schedules for gas metal-arc
welding of aluminum.

GMAW Aluminum Weld

An aluminum weld completed using the GMAW process. The welder “lays a bead” of molten metal that becomes a slag free weld.

Aluminum Welding Technique

The electrode wire must be
clean. The arc is struck with the electrode wire protruding about 1/2
in. (12.7 mm) from the cup. A frequently used technique is to strike the
arc approximately 1.0 in. (25.4 mm) ahead of the beginning of the weld
and then quickly bring the arc to the weld starting point, reverse the
direction of travel, and proceed with normal welding. Alternatively, the
arc may be struck outside the weld groove on a starting tab. When
finishing or terminating the weld, a similar practice may be followed by
reversing the direction of welding, and simultaneously increasing the
speed of welding to taper the width of the molten pool prior to breaking
the arc. This helps to avert craters and crater cracking. Runoff tabs
are commonly used. Having established the arc, the welder moves the
electrode along the joint while maintaining a 70 to 85 degree forehand
angle relative to the work.

A string bead technique is normally
preferred. Care should be taken that the forehand angle is not changed
or increased as the end of the weld is approached. Arc travel speed
controls the bead size. When welding aluminum with this process, it is
must important that high travel speeds be maintained. When welding
uniform thicknesses, the electrode to work angle should be equal on both
sides of the weld. When welding in the horizontal position, best
results are obtained by pointing the gun slightly upward. When welding
thick plates to thin plates, it is helpful to direct the arc toward the
heavier section. A slight backhand angle is sometimes helpful when
welding thin sections to thick sections. The root pass of a joint
usually requires a short arc to provide the desired penetration.
Slightly longer arcs and higher arc voltages may be used on subsequent
passes.

The wire feeding equipment for aluminum welding must be
in good adjustment for efficient wire feeding. Use nylon type liners in
cable assemblies. Proper drive rolls must be selected for the aluminum
wire and for the size of the electrode wire. It is more difficult to
push extremely small diameter aluminum wires through long gun cable
assemblies than steel wires. For this reason, the spool gun or the newly
developed guns which contain a linear feed motor are used for the small
diameter electrode wires. Water-cooled guns are required except for
low-current welding. Both the constant current (CC) power source with
matching voltage sensing wire feeder and the constant voltage (CV) power
source with constant speed wire feeder are used for welding aluminum.
In addition, the constant speed wire feeder is sometimes used with the
constant current power source. In general, the CV system is preferred
when welding on thin material and using all diameter electrode wire. It
provides better arc starting and regulation. The CC system is preferred
when welding thick material using larger electrode wires. The weld
quality seems better with this system. The constant current power source
with a moderate drop of 15 to 20 volts per 100 amperes and a constant
speed wire feeder provide the most stable power input to the weld and
the highest weld quality.

Aluminum Welding Joint design

Edges may be
prepared for welding by sawing, machining, rotary planing, routing or
arc cutting.

Gas Tungsten-Arc (TIG) Welding (GTAW)

Precautions

The gas tungsten arc welding process is used for welding the thinner
sections of aluminum and aluminum alloys. There are several precautions
that should be mentioned with respect to using this process.

Alternating current is recommended for
general-purpose work since it provides the half-cycle of cleaning
action. Table 7-22 provides welding procedure schedules for using the
process on different thicknesses to produce different welds. AC welding,
usually with high frequency, is widely used with manual and automatic
applications. Procedures should be followed closely and special
attention given to the type of tungsten electrode, size of welding
nozzle, gas type, and gas flow rates. When manual welding, the arc
length should be kept short and equal to the diameter of the electrode.
The tungsten electrode should not protrude too far beyond the end of the
nozzle. The tungsten electrode should be kept clean. If it does
accidentally touch the molten metal, it must be redressed.

Aluminum Welding Welding power sources designed for the gas tungsten
arc welding process should be used. The newer equipment provides for
programming, pre-and post-flow of shielding gas, and pulsing.

Aluminum Welding For automatic or machine welding,
direct current electrode negative (straight polarity) can be used.
Cleaning must be extremely efficient, since there is no cathodic
bombardment to assist. When dc electrode negative is used, extremely
deep penetration and high speeds can be obtained. Table 7-23 lists
welding procedure schedules for dc electrode negative welding.

The Aluminum Welding shielding gases are argon, helium, or a mixture
of the two. Argon is used at a lower flow rate. Helium increases
penetration, but a higher flow rate is required. When filler wire is
used, it must be clean. Oxide not removed from the filler wire may
include moisture that will produce polarity in the weld deposit.

Manual MIG Aluminum Weld

Alternating Current Welding

Characteristics of Process

Aluminum welding with the gas tungsten-arc welding process using alternating
current produces an oxide cleaning action. Argon shielding gas is used.
Better results are obtained when welding aluminum with alternating
current by using equipment designed to produce a balanced wave or equal
current in both directions. Unbalance will result in loss of power and a
reduction in the cleaning action of the arc. Characteristics of a
stable arc are the absence of snapping or cracking, smooth arc starting,
and attraction of added filler metal to the weld puddle rather than a
tendency to repulsion. A stable arc results in fewer tungsten
inclusions.

MIG Manual Aluminum Weld

Aluminum Welding Technique

For manual welding of
aluminum with ac, the electrode holder is held in one hand and filler
rod, if used, in the other. An initial arc is struck on a starting block
to heat the electrode. The arc is then broken and reignited in the
joint. This technique reduces the tendency for tungsten inclusions at
the start of the weld. The arc is held at the starting point until the
metal liquefies and a weld pool is established. The establishment and
maintenance of a suitable weld pool is important, and welding must not
proceed ahead of the puddle.

If filler metal is required, it may be
added to the front or leading edge of the pool but to one side of the
center line. Both hands are moved in unison with a slight backward and
forward motion along the joint. The tungsten electrode should not touch
the filler rod. The hot end of the filler rod should not be withdrawn
from the argon shield. A short arc length must be maintained to obtain
sufficient penetration and avoid undercutting, excessive width of the
weld bead, and consequent loss of penetration control and weld contour.

One rule is to use an arc length approximately equal to the diameter of
the tungsten electrode. When the arc is broken, shrinkage cracks may
occur in the weld crater, resulting in a defective weld. This defect can
be prevented by gradually lengthening the arc while adding filler metal
to the crater. Then, quickly break and restrike the arc several times
while adding additional filler metal to the crater, or use a foot
control to reduce the current at the end of the weld. Tacking before
welding is helpful in controlling distortion. Tack welds should be of
ample size and strength and should be chipped out or tapered at the ends
before welding over.

Welding Joint Design

Joint designs
are applicable to the gas tungsten-arc welding
process with minor exceptions. Inexperienced welders who cannot maintain
a very short arc may require a wider edge preparation, included angle,
or joint spacing. Joints may be fused with this process without the
addition of filler metal if the base metal alloy also makes a
satisfactory filler alloy. Edge and corner welds are rapidly made
without addition of filler metal and have a good appearance, but a very
close fit is essential.

Direct Current Straight Polarity

Characteristics of the Process

This process, using
helium and thoriated tungsten electrodes is advantageous for many
automatic welding operations, especially in the welding of heavy
sections. Since there is less tendency to heat the electrode, smaller
electrodes can be used for a given welding current. This will contribute
to keeping the weld bead narrow. The use of direct current straight
polarity (dcsp) provides a greater heat input than can be obtained with
ac current. Greater heat is developed in the weld pool, which is
consequently deeper and narrower.

Techniques

A high frequency
current should be used to initiate the arc. Touch starting will
contaminate the tungsten electrode. It is not necessary to form a puddle
as in ac welding, since melting occurs the instant the arc is struck.
Care should be taken to strike the arc within the weld area to prevent
undesirable marking of the material. Standard techniques such as runoff
tabs and foot operated heat controls are used. These are helpful in
preventing or filling craters, for adjusting the current as the work
heats, and to adjust for a change in section thickness. In dcsp welding,
the torch is moved steadily forward. The filler wire is fed evenly into
the leading edge of the weld puddle, or laid on the joint and melted as
the arc roves forward. In all cases, the crater should be filled to a
point above the weld bead to eliminate crater cracks. The fillet size
can be controlled by varying filler wire size. DCSP is adaptable to
repair work. Preheat is not required even for heavy sections, and the
heat affected zone will be smaller with less distortion.

Aluminum Welding Joint designs

For manual dcsp, the
concentrated heat of the arc gives excellent root fusion. Root face can
be thicker, grooves narrower, and build up can be easily controlled by
varying filler wire size and travel speed.

Square Wave Alternating Current Welding (TIG)

Techniques

A high frequency
current should be used to initiate the arc. Touch starting will
contaminate the tungsten electrode. It is not necessary to form a puddle
as in ac welding, since melting occurs the instant the arc is struck.
Care should be taken to strike the arc within the weld area to prevent
undesirable marking of the material. Standard techniques such as runoff
tabs and foot operated heat controls are used. These are helpful in
preventing or filling craters, for adjusting the current as the work
heats, and to adjust for a change in section thickness. In dcsp welding,
the torch is moved steadily forward. The filler wire is fed evenly into
the leading edge of the weld puddle, or laid on the joint and melted as
the arc roves forward. In all cases, the crater should be filled to a
point above the weld bead to eliminate crater cracks. The fillet size
can be controlled by varying filler wire size. DCSP is adaptable to
repair work. Preheat is not required even for heavy sections, and the
heat affected zone will be smaller with less distortion.

Aluminum Welding Joint designs

For manual dcsp, the
concentrated heat of the arc gives excellent root fusion. Root face can
be thicker, grooves narrower, and build up can be easily controlled by
varying filler wire size and travel speed.

Shielded Metal-Arc Welding

In the shielded metal-arc
welding process, a heavy dipped or extruded flux coated electrode is
used with dcrp. The electrodes are covered similarly to conventional
steel electrodes. The flux coating provides a gaseous shield around the
arc and molten aluminum puddle, and chemically combines and removes the
aluminum oxide, forming a slag. When welding aluminum, the process is
rather limited due to arc spatter, erratic arc control, limitations on
thin material, and the corrosive action of the flux if it is not removed
properly.

Shielded Carbon-Arc Welding

The shielded carbon-arc
welding process can be used in joining aluminum. It requires flux and
produces welds of the same appearance, soundness, and structure as those
produced by either oxyacetylene or oxy-hydrogen welding. Shielded
carbon-arc welding is done both manually and automatically. A carbon arc
is used as a source of heat while filler metal is supplied from a
separate filler rod. Flux must be removed after welding; otherwise
severe corrosion will result. Manual shielded carbon-arc welding is
usually limited to a thickness of less than 3/8 in. (9.5 mm),
accomplished by the same method used for manual carbon arc welding of
other material. Joint preparation is similar to that used for gas
welding. A flux covered rod is used.

Atomic Hydrogen Welding

This welding process consists
of maintaining an arc between two tungsten electrodes in an atmosphere
of hydrogen gas. The process can be either manual or automatic with
procedures and techniques closely related to those used in oxyacetylene
welding. Since the hydrogen shield surrounding the base metal excludes
oxygen, smaller amounts of flux are required to combine or remove
aluminum oxide. Visibility is increased, there are fewer flux
inclusions, and a very sound metal is deposited.

Stud Welding

Aluminum stud welding may be accomplished with
conventional arc stud welding equipment, using either the capacitor
discharge or drawn arc capacitor discharge techniques. The conventional
arc stud welding process may be used to weld aluminum studs 3/16 to 3/4
in. (4.7 to 19.0 mm) diameter. The aluminum stud welding gun is modified
slightly by the addition of a special adapter for the control of the
high purity shielding gases used during the welding cycle. An added
accessory control for controlling the plunging of the stud at the
completion of the weld cycle adds materially to the quality of weld and
reduces spatter loss. Reverse polarity is used, with the electrode gun
positive and the workpiece negative. A small cylindrical or cone shaped
projection on the end of the aluminum stud initiates the arc and helps
establish the longer arc length required for aluminum welding.

Processes

The unshielded capacitor discharge or drawn arc
capacitor discharge stud welding processes are used with aluminum studs
1/16 to 1/4 in. (1.6 to 6.4 mm) diameter. Capacitor discharge welding
uses a low voltage electrostatic storage system, in which the weld
energy is stored at a low voltage in capacitors with high capacitance as
a power source. In the capacitor discharge stud welding process, a
small tip or projection on the end of the stud is used for arc
initiation. The drawn arc capacitor discharge stud welding process uses a
stud with a pointed or slightly rounded end. It does not require a
serrated tip or projection on the end of the stud for arc initiation. In
both cases, the weld cycle is similar to the conventional stud welding
process. However, use of the projection on the base of the stud provides
the most consistent welding. The short arcing time of the capacitor
discharge process limits the melting so that shallow penetration of the
workpiece results. The minimum aluminum work thickness considered
practical is 0.032 in. (0.800 mm).

Electron Beam Welding

Electron beam welding is a fusion joining process
in which the workpiece is bombarded with a dense stream of high
velocity electrons, and virtually all of the kinetic energy of the
electrons is transformed into heat upon impact. Electron beam welding
usually takes place in an evacuated chamber. The chamber size is the
limiting factor on the weldment size. Conventional arc and gas heating
melt little more than the surface. Further penetration comes solely by
conduction of heat in all directions from this molten surface spot. The
fusion zone widens as it depends. The electron beam is capable of such
intense local heating that it almost instantly vaporizes a hole through
the entire joint thickness. The walls of this hole are molten, and as
the hole is moved along the joint, more metal on the advancing side of
the hole is melted. This flaws around the bore of the hole and
solidifies along the rear side of the hole to make the weld. The
intensity of the beam can be diminished to give a partial penetration
with the same narrow configuration. Electron beam welding is generally
applicable to edge, butt, fillet, melt-thru lap, and spot welds. Filler
metal is rarely used except for surfacing.

Welding Resistance Welding

The aluminum welding resistance welding
processes (spot, seam, and flash welding) are important in fabricating
aluminum alloys. These processes are especially useful in joining the
high strength heat treatable alloys, which are difficult to join by
fusion welding, but can be joined by the resistance welding process with
practically no loss in strength. The natural oxide coating on aluminum
has a rather high and erratic electrical resistance. To obtain spot or
seam welds of the highest strength and consistency, it is usually
necessary to reduce this oxide coating prior to welding.

Welding Spot Welding

Welds of uniformly
high strength and good appearance depend upon a consistently low surface
resistance between the workplaces. For most applications, some cleaning
operations are necessary before spot or seam welding aluminum. Surface
preparation for welding generally consists of removal of grease, oil,
dirt, or identification markings, and reduction and improvement of
consistency of the oxide film on the aluminum surface. Satisfactory
performance of spot welds in service depends to a great extent upon
joint design. Spot welds should always be designed to carry shear loads.
However, when tension or combined loadings may be expected, special
tests should be conducted to determine the actual strength of the joint
under service loading. The strength of spot welds in direct tension may
vary from 20 to 90 percent of the shear strength.

Seam Welding

Seam welding of
aluminum and its alloys is very similar to spot welding, except that the
electrodes are replaced by wheels. The spots made by a seam welding
machine can be overlapped to form a gas or liquid tight joint. By
adjusting the timing, the seam welding machine can produce uniformly
spaced spot welds equal in quality to those produced on a regular spot
welding machine, and at a faster rate. This procedure is called roll
spot or intermittent seam welding.

Aluminum Flash Welding

All aluminum alloys
may be joined by the flash welding process. This process is
particularly adapted to making butt or miter joints between two parts of
similar cross section. It has been adapted to joining aluminum to
copper in the form of bars and tubing. The joints so produced fail
outside of the weld area when tension loads are applied.

Aluminum Gas Welding

Gas welding has been
done on aluminum using both oxyacetylene and oxyhydrogen flames. In
either case, an absolutely neutral flame is required. Flux is used as
well as a filler rod. The process also is not too popular because of low
heat input and the need to remove flux.

Electroslag Welding

Electroslag
welding is used for joining pure aluminum, but is not successful for
welding the aluminum alloys. Submerged arc welding has been used in some
countries where inert gas is not available.

Other Processes

Most of the solid
state welding processes, including friction welding, ultrasonic welding,
and cold welding are used for aluminums. Aluminum can also be joined by
soldering and brazing. Brazing can be accomplished by most brazing
methods. A high silicon alloy filler material is used.